This invention relates to a constant velocity universal joint.
A constant velocity universal joint is capable of transmitting torque at a uniform angular velocity from a drive shaft to a driven shaft which crosses the drive shaft, and is characterized by the fact that it has a very large maximum joint working angle in view of its construction. In construction, a constant velocity universal joint comprises an outer race located at the end of one shaft and having a plurality of ball rolling grooves formed on its inner circumferential surface, an inner race located at the end of the other shaft and having a plurality of ball rolling grooves formed on its outer circumferential surface, and a cage for holding in a predetermined position a ball disposed between the ball rolling grooves of the outer and inner races. The ball rolling grooves formed on the circumferential surfaces of the two races may have their centers displaced for an equal distance to right and left respectively from the center of the angle of the joint, so as to thereby control the movement of the cage and the ball and cause the ball to be positioned on a plane bisecting the angle formed by the drive shaft and the driven shaft at all times. This arrangement is conductive to the prevention of variations in the number of revolutions and torque.
Also, in some applications, the ball rolling grooves may be shaped such that they are elliptically arcuate in their transverse sectional surfaces to prevent the ball accidentally riding over the end portions of the grooves and damaging same when the ball rolling grooves are of a circularly arcuate shape. In this application, the ball joint is designed such that it can well withstand an impact load applied abruptly.
In this type of constant velocity universal joint of the prior art, the outer race and the inner race have the same radius of curvature at the transverse sectional surface of the ball rolling groove at the point of contact between the ball and the ball rolling groove. When one considers the contact stress between the ball and the ball rolling groove, the contact stress .sigma..sub.c can be expressed in the following relation: .sigma..sub.c .alpha. (a load applied to the ball).sup.1/3. It is known in the theory of elasticity that the contact stress .sigma..sub.c is low when the ball rolling groove consists of concave surface portions and high when it consists of convex surface portions. Thus it is not desirable in working on the outer race groove that the outer race and the inner race have the same radius of curvature at the transverse sectional surface of the ball rolling groove at the point of contact between the ball and the ball rolling groove. The concave and convex surface portions of a ball rolling groove have a characteristic such that when the inner and outer races have the same radius of curvature at the transverse sectional surface of the ball rolling groove, the contact stress of the ball rolling groove of the outer race becomes lower than that of the inner race. Thus one has only to provide means for rendering the contact stresses of the two grooves equal to each other. The end can be attained by increasing the radius of curvature at the transverse sectional surface of the ball rolling groove of the outer race as compared with that of the inner race. This facilitates working or machining of the ball joint to achieve the desired precision finishes.